Method for generating CNC machine offset based on thermal model

ABSTRACT

The present disclosure generally describes a method for processing a workpiece in a machine, where the method determines an offset of the machine and adjusts for the offset during production operation. In one form, the method includes logging offset data of the machine over a period of operational time having varying thermal conditions, and comparing the logged offset data against a thermal model, where the thermal model is generated based on a probing routine and dry cycling for a plurality of test cycles on a calibration artifact. Based on the comparing, the method estimates offsets for the machine and adjusts offsets of the machine during operation.

FIELD

The present disclosure relates to a method for determining an offset ina CNC machine due to thermal growth of one or more components.

BACKGROUND

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

Generally, computer numerical control (CNC) machines executepreprogrammed sequence of commands to automate various machiningoperations. For example, drills, lathes, and water jet cutters can beconfigured as CNC machines. A CNC machine typically includes multiplecomponents such as a motor, spindle, ballscrew, rotary axes, and column,and may be operable to orientate a work piece relative to a toolattached to the spindle before machining the work piece. Each of thesecomponents have different thermal expansion properties and can cause theCNC machine to become out of position.

To properly position the tool with a work piece, the CNC machine iscalibrated with a machine reference point (“reference point”hereinafter) that serves as the origin point of the coordinate systemused by the CNC machine. The reference point may be determined using aprobing routine that utilizes a precision gage bore, which can bepositioned on a fixture, and a touch probe attached to the spindle.During machining, the temperature can fluctuate due to heat caused by,for example, motor, cutting energy, idle operation and friction. Thefluctuation in temperature can lead to thermal growth of variouscomponents, such as the spindle, the ballscrew, the rotary axes, and thecolumn, and cause the reference point to shift. For example, in minimumquantity lubrication (MQL) machining, where flood coolant is minimal,thermal growth can be up to 100 μm. In high-volume production, frequentstarts and stops of a machine occur due to various reasons, such as amachine being blocked, starved, a gantry issue, and shift change, whichcan also move the reference point due to thermal growth.

These offset adjustment issues, among other issues related to theperformance and cycle time of CNC machine, are addressed by the presentdisclosure.

SUMMARY

In one form, the present disclosure provides for a method forcompensating for thermal variations in a machine. The method includesinstrumenting the machine with a plurality of temperature sensors,enclosing the machine in an environmentally controllable atmosphere, andmounting a calibration artifact into the machine. The method furtherincludes soaking the machine at a plurality of predetermined ambienttemperatures, probing the calibration artifact at the plurality oftemperatures, and generating a thermal model of the machine based on theprobing.

In another form, the present disclosure provides for a method forprocessing a workpiece in a machine. The method includes logging offsetdata of the machine over a period of operational time having varyingthermal conditions, comparing the logged offset data against a thermalmodel, where the thermal model is generated based on a probing routineand dry cycling for a plurality of test cycles on a calibrationartifact. Based on the comparing, the method estimates offsets for themachine and adjusts offsets of the machine during operation.

In yet another form, the present disclosure provides for a method forprocessing a workpiece in a CNC machine. The method includes loggingoffset data of the CNC machine during production operation over a periodof time having varying environmental conditions. The method compares thelogged offset data against a thermal model of the CNC machine duringproduction operation, estimates offsets for the CNC machine based on thecomparing; and adjusts offsets of the machine during productionoperation such that the CNC machine is adjusted without experiencingdowntime.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

DRAWINGS

In order that the disclosure may be well understood, there will now bedescribed various forms thereof, given by way of example, referencebeing made to the accompanying drawings, in which:

FIG. 1 is schematic view of a CNC machine having a controllerconstructed according to the teachings of the present disclosure;

FIG. 2 is a block diagram of the controller of the CNC machine of FIG.1;

FIG. 3 illustrates a thermal characterization process according to theteachings of the present disclosure;

FIG. 4 is a flowchart of an example thermal characterization processaccording to the teachings of the present disclosure; and

FIG. 5 is a flowchart of an example of a thermal offset calibrationprocess according to the teachings of the present disclosure.

The drawings described herein are for illustration purposes only and arenot intended to limit the scope of the present disclosure in any way. Itshould be understood that throughout the drawings, correspondingreference numerals indicate like or corresponding parts and features.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is notintended to limit the present disclosure, application, or uses.

In a manufacturing environment, CNC machines are generally required toperform one or more machining operations on a work piece within adesignated cycle time. To address the shift in the reference point,manufacturers of CNC machines may interrupt machining operations toperform gage bore probing to identify the thermal growth and compensateaccordingly. While gage probing may address the shift, it can take upcycle time and is typically considered a non-value add operation inmanufacturing, and yet it considers only one or a limited number ofcutting planes. Accordingly, frequent probing increases cycle time andis a disadvantage in mass production.

The present disclosure is directed toward a method for compensating foran offset of the CNC machine using a predetermined thermal model, anddoes not require high precision gage probing that may take up valuablemachining time. Specifically, a thermal offset calibration process ofthe present disclosure is stored and executed by a controller of the CNCmachine to estimate an offset of the CNC machine based on a thermalmodel stored in the CNC machine and on other data, such as temperatureand stored offset(s). The process may then adjust the CNC machine sothat the origin of the coordinate system aligns with the originalreference point. A detailed explanation of the thermal offsetcalibration will now be provided with reference to drawings.

Referring to FIG. 1, a CNC machine 100 is shown that includes a table102 having a precision gage bore 103, a spindle 104, a trunnion 105, atool 106 disposed at the end of the spindle 104, a controller 108, oneor more user interfaces 110 that is communicably coupled to thecontroller 108, and one or more temperature sensors 112 according to theteachings of the present disclosure. The spindle 104 is generallyoperable to move relative to a work piece 114 disposed on the table 102along the X, Y, Z axes. The part is generally operable to orient usingA, B axes to present the part face/plane that needs machined. The tool106 may be operable to rotate about the Z-axis and apply a force ontothe work piece 114 to perform a machine operation, such as drilling borethrough the work piece 114. The CNC machine 100 may be configured tohave multiple tools such that the spindle 104 is operable to switchbetween the tools in between non-machining operations.

The temperature sensors 112 measure ambient temperature and may bepositioned at different locations of the CNC machines 100, such as thespindle 104, the table 102, machine base (not shown), ballscrew (notshown), and other suitable locations, which may be determined accordingto a method of the present disclosure as set forth in greater detailbelow. The temperature sensor 112 may output data to the controller 108at a fixed interval throughout machining and non-machining operations.Alternatively, the temperature sensors 112 may output data at a fixedoperation time such as at the start and stop of designated operations,and/or when requested by the controller 108.

The CNC machine 100 may be operable by a user, such a machine operator,via the user interface 110. In the illustrated form, the user interface110 includes a monitor 116 that may display a graphical user interface,and a keyboard 118. The user interface 110 may include other components,such as a mouse, a touchscreen display, and other suitable devices foroperating the CNC machine 100.

The controller 108 controls the operation of the CNC machine 100 basedon inputs received and on predetermined programs stored and executed bythe controller 108. The controller 108 may be in communication withvarious external devices via wired and wireless communication. Forexample, the controller 108 may be communicably coupled to the userinterfaces 110 and the temperatures sensors 112 by way of acommunication port and cable and/or through wireless communication byway of a transceiver (e.g., Bluetooth, ZigBee, and/or Wi-Fi). Thecontroller 108 may communicate with other external devices such asservers located external of the CNC machine 100, and external memory,among others.

With reference to FIG. 2, the controller 108 may include memory 202(e.g., RAM, ROM, EPROM, and/or EEPROM) that stores control programs, anda processor 208 for executing the programs in the memory 202. Some ofthe programs are directed toward one or more machining operations to beperformed by the CNC machine 100, and may control the CNC machine 100to, for example, place the work piece 114 on the table 102, machine thework piece, rotate the work piece 114, and move the work piece 114 fromthe table 102. Other programs may be directed toward non-machiningoperations, such as the thermal offset calibration process, to adjustfor an offset caused by thermal growth of one or more components of theCNC machine 100, as described in detail herein. In addition to thecontrol programs, the memory 202 may also store other information, suchas data inputted by the user via the user interfaces 110, temperaturedetected by the temperature sensors 112, and prestored data such ascalibrated position information of the gage bore 103 and a thermalmodel.

As described above, components of the CNC machine 100 may experiencethermal growth causing the reference point of the machine coordinatesystem to shift. To compensate for this shift, the controller 108executes the thermal offset calibration process in which the controller108 estimates an offset of the CNC machine 100 using a predeterminedthermal model and logged offset data, and then adjusts for offset suchthat the work piece is aligned relative to, for example, the tool 106based on the reference point.

The thermal model characterizes the thermal growth of the CNC machine100, and is used to estimate machine growth offset along one or moreaxes at a given temperature, and machine state (e.g., warm-up time, andbreak time, etc). One factor in formulating the thermal model includesevaluating the effect that temperature has on the thermal growth of oneor more components of the CNC machine 100. To do this, the CNC machine100 may undergo one or more thermal based experiments in which theenvironmental condition of the CNC machine 100 is controlled and thethermal growth of key components are measured.

FIG. 3 illustrates a thermal characterization process, and FIG. 4 is aflowchart of an example thermal characterization process. The thermalcharacterization process may be performed at, for example, the originalmanufacture of the CNC machine 100 or at the manufacturing facilityutilizing the CNC machine 100. In either situation, the characterizationis performed before the thermal offset calibration process. In addition,before the thermal characterization is conducted, a high precision gageprobing may be performed to calibrate the reference point of the CNCmachine 100.

To monitor temperatures, the CNC machine 100 is instrumented withvarious sensors such as multiple temperature sensors placed at variouspositions about the machine 100, at 402. To effectively regulate theambient temperature, the CNC machine 100, at 404, may be enclosed in anenvironmentally controllable atmosphere such as a tent (image 300 ofFIG. 3).

A calibration artifact, such a rectangular block shown in image 302 ofFIG. 3, may be mounted in the CNC machine 100, at 406. The calibrationartifact has precise dimensions that are known, and may be formed ofmaterial having low coefficient of thermal expansion. The artifact mayinclude multiple bores on different faces of the artifact.

The CNC machine 100 and the artifact are then soaked at one or morepredetermined ambient temperatures and the artifact is probed at eachtemperature to ascertain the location of one or more bores, at 408.Specifically, the CNC machine 100 undergoes an artifact test, at 408, inwhich the ambient temperature is adjusted to simulate differentenvironmental conditions, such as winter, spring, summer and fall. Theartifact test may also include probing the artifact at differenttemperatures and different machine states to measure the location of thebore center and/or the bore depth of one or more of the bores along theartifact (e.g., image 302 of FIG. 3 illustrates an artifact beingprobed). The test may also measure other positions, such as the top andbottom sides of the artifact.

The artifact test may also execute a dry cycle in which the CNC machine100 is controlled to carry out one or more machining operations withouta work piece and tool. That is, the CNC machine 100 simulates machiningoperations which may include rotating the spindle 104 and relocating awork piece by performing the required movement without the work piece,all of which can affect the thermal growth of the components. Inaddition to the machining operations, the dry cycle may also includenon-machining operations such as tool changes, rapid feeds, and/or A/Bindexing of different angular index positions and combinations of thetrunnion 105 (A-axis) and the table 102 (B-axis) that sits on thetrunnion 105. After the dry cycle, the artifact test probes the artifactagain. Addition detail regarding such temperature controlled artifacttest for characterizing thermal growth is described in U.S. patentapplication Ser. No. 14/463,988, which is commonly assigned with thepresent application and the contents of which are incorporated herein byreference in its entirety.

The artifact test may provide insight in the thermal growth of, forexample, sensitive components of the CNC machine 100, and foridentifying locations about the CNC machine 100 that are ideal foraccurately detecting the ambient temperature. For example, graph 304 inFIG. 3 illustrates an expected thermal growth from a thermal study inwhich a temperature compensation, which is used to adjust for anyoffset, is turned off for one data set and turned on for another dataset. As shown, without any thermal compensation the thermal growthoffset varies significantly from the true measurement.

The artifact test may also be used to generate subsystem thermal modelsfor sensitive components, such as spindle, trunnion, ballscrew, whichcan provide further understanding in the course thermal growth of suchcomponents. For example, a cutting spindle, which takes the directcutting load, may heat up at quickly than other components, and as aresult, grows as the temperature increases. The cutting spindle may bestudied by placing one or more sensors on the spindle, exercising onlythe spindle, and then developing a spindle only thermal model based onthe data collected. Probe or other systems can be used to measurespindle growth. Other machine components such as part table, fixture,column and bed, which are not directly involved in cutting, may bestudied by soaking them in different ambient temperatures.

Using the data gathered from the artifact test, a thermal model isgenerated and stored in the controller 108 of the CNC machine 100, at410. The model includes the thermal growth of the CNC machine 100, whichis determined as growth offsets measured along, for example, a Cartesiancoordinate system based on the locations measured on the artifact (e.g.,bore center and depth) during the artifact test and on known locationsof the artifact. For example, the difference between each measuredlocation and its respective known location is identified as a growthoffset for that bore. The growth offset is associated with thetemperature and machine state associated with the measured location. Thethermal model associates the thermal growth, the temperature, and time(e.g., plot system 306 of FIG. 3), and is used to estimate machinegrowth offset at a given temperature and known machine state (e.g.,warm-up, steady state, cool down stage, and break-time).

When the CNC machine 100 is in operation, the controller 108 routinelyperforms the thermal offset calibration process to correct the offset ofthe CNC machine 100. The frequency at which the controller 108 executesmay be dependent on the environmental and operating state that the CNCmachine 100 has undergone. Specifically, the process may be performedwhen the CNC machine 100 has experienced various operating scenarios,such as starts, stoppages, continuous run time, and temperature swings.Thus, if the CNC machine 100 experiences such scenarios after two daysof operation, the frequency can be set for every week or every month.The frequency may also be adjusted via the user interface, andtherefore, is customized for each CNC machine.

FIG. 5 provides an example of a thermal offset calibration processexecuted by the controller 108, and in one form of the presentdisclosure, is performed at a non-machining operation state of the CNCmachine 100 during production, such as retooling, steady state, etc.That is, the CNC machine 100 is not taken off-line like in the highprecision gage probing, and is adjusted during a non-machining operationof production.

At 502 of FIG. 5, the controller 108 determines an offset of theprecision gage bore 103 relative to a component of the CNC machine 100.For example, the controller 108 executes a probing routine to determinethe location of the bore 103 (e.g., X, Y, Z positions) relative to thespindle 104 and saves the location as a measured location in the memory202. The controller 108 then determines the offset of the bore 103relative to the spindle 104 by subtracting the measured location from aknown location stored in the memory 202. While the offset data is basedon the spindle 104, other components of the CNC machine, such asballscrew, trunnion, column, and table, may also be used.

The controller 108 then acquires the ambient temperature from the one ormore temperature sensors 112 and a machine state of the CNC machine 100,and stores or logs the offset data determined at 502 with thetemperature and machine state acquired in the memory 202, as an offsetrecord, at 504. The controller 108 may continue to store the offsetrecord even after the process is complete for use for future offsetcalibration processes. The machine state may be inputted by the machineoperator via the user interface.

At 506, the controller 108 compares the logged offset record(s) againstthe thermal model based on a correlation analysis model. In an exampleembodiment, the controller 108 uses regression analysis as thecorrelation analysis model. Regression analysis is a known statisticalprocess that can be used to estimate the relationship between adependent variable and one or more independent variables. For example,here, the dependent variable may be the offsets of the reference pointof the CNC machine 100 along the positioning axes (e.g. X, Y, Z), andthe independent variables may include temperature and machine state. Thecontroller 108 may also use other methods for comparing the loggedoffset record against the thermal model. For example, in addition to orin lieu of regression analysis, the controller 108 may use principalcomponent analysis, look-up tables, and other suitable data analyticsmethods.

At 508, the controller 108 estimates the offsets of the CNC machine 100and the adjusts for the offsets during the machining operation. Forexample, if using regression analysis, the controller 108 generatesregression equations that are used to estimate the offsets of the CNCmachine 100 along the positioning axes (e.g. X, Y, Z). For instance,equations 1 to 3 are example equations for determining the offsets basedon the temperatures of the bed and spindle (i.e., T_(bed) and T_(spi)).With the estimated offsets, the controller 108 is able to compensate forthe shift in the reference point during normal machining operations. Forexample, the controller 108 calculates the offsets and applies them byadjustments of positions in a CNC program stored in the controller 108.It should be readily understood that the specific coefficients andconstants provided in equations 1 to 3 are dependent upon the offsetsmeasured, and therefore, the present disclosure should not be restrictedto the specific values provided in the equations.x-offset=0.0400−0.00160T _(bed)+0.00303T _(spi) R ²=0.85  Equation 1y-offset=0.300−0.02357T _(bed)+0.00121T _(spi) R ²=0.955  Equation 2z-offset=0.1661−0.00734T _(bed)+0.00541T _(spi) R ²=0.879  Equation 3

Based on the foregoing, the thermal offset calibration processcompensates for the shift in the CNC machine without having to performthe time consuming high precision gage probing process and substantiallyreduces the cycle time needed to correct the reference point. That is,in one form of the present disclosure, the thermal offset calibrationprocess is performed during a non-machining operation of production,such as tool change. The temperature compensation routine describedherein utilizes a predetermined thermal model for estimating the offsetin lieu of performing high cost probing routine.

It should be readily understood that the steps described with regard tothe processes illustrated in FIGS. 4 and 5 may be modified and shouldnot be limited to the example provided herein. For example, with regardto FIG. 4, the artifact may be positioned in the CNC machine before theCNC machine is enclosed in environmentally controllable atmosphere, andin FIG. 5, the controller may be configured to estimate the offset andadjust the CNC machine after a certain number of measured offset recordsare first logged into the system.

The description of the disclosure is merely exemplary in nature and,thus, variations that do not depart from the substance of the disclosureare intended to be within the scope of the disclosure. Such variationsare not to be regarded as a departure from the spirit and scope of thedisclosure.

What is claimed is:
 1. A method of compensating for thermal variationsin a machine comprising: instrumenting the machine with a plurality oftemperature sensors; enclosing the machine in an environmentallycontrollable atmosphere; mounting a calibration artifact into themachine, wherein the calibration artifact has precise dimensions thatare known; soaking the machine at a plurality of predeterminedtemperatures; probing the calibration artifact at the plurality oftemperatures; and generating a thermal model of the machine based on theprobing.
 2. The method according to claim 1, wherein the probingcomprises a probing routine and dry cycling for at least one test cycle.3. The method according to claim 1, wherein the plurality ofpredetermined temperatures simulates four (4) seasons of winter, spring,summer, and fall.
 4. The method according to claim 1, wherein thecalibration artifact is a material with low coefficient of thermalexpansion.
 5. The method according to claim 1, wherein the instrumentingcomprises instrumenting components of the machine and generating athermal model of the components.
 6. The method according to claim 1,wherein the machine is a CNC machine.
 7. The method according to claim6, wherein the instrumenting comprises instrumenting at least onecomponent of the CNC machine and generating a thermal model of the atleast one component.
 8. The method according to claim 7, wherein thecomponent is selected from the group consisting of a spindle, aballscrew, a part table, a trunnion, a column, a bed, and a fixture. 9.The method according to claim 6, further comprising: logging offset dataof the CNC machine during production operation over a period of timehaving varying environmental conditions; comparing the logged offsetdata against the thermal model of the CNC machine during productionoperation; estimating offsets for the CNC machine based on thecomparing; and adjusting offsets of the CNC machine during productionoperation such that the CNC machine is adjusted without experiencingdowntime.
 10. The method according to claim 9, wherein the logged offsetdata is based on at least one component of the CNC machine, and the atleast one component is selected from the group consisting of a spindle,a ballscrew, a part table, a trunnion, a column, a bed, and a fixture.11. The method according to claim 9, wherein positions of thetemperature sensors are adjusted based on the comparing and the thermalmodel.
 12. The method according to claim 9, wherein additionaltemperature sensors are positioned on the CNC machine based on thecomparing and the thermal model.
 13. The method according to claim 1,wherein the thermal model includes machine growth in a multi-axiscoordinate system.
 14. The method according to claim 1, furthercomprising: logging offset data of the machine over a period ofoperational time having varying thermal conditions; comparing the loggedoffset data against the thermal model; estimating offsets for themachine based on the comparing; and adjusting offsets of the machineduring operation.
 15. The method according to claim 14, wherein thecomparing is conducted using at least one of regression analysis,principal component analysis, and look-up tables.
 16. The methodaccording to claim 14, wherein the machine is a CNC machine, and thelogged offset data is based on at least one component of the CNCmachine.
 17. The method according to claim 16, wherein the component isselected from the group consisting of a spindle, a ballscrew, a parttable, a trunnion, a column, a bed, and a fixture.
 18. The methodaccording to claim 14, wherein positions of the temperature sensors areadjusted based on the comparing and the thermal model.
 19. The methodaccording to claim 14, wherein the offsets are based on a multi-axiscoordinate system.
 20. A part manufactured according to the method ofclaim 14.